In some power system loads requiring a DC voltage, the power equipment used therein has operating characteristics that are desirable to avoid, in certain circumstances. An example is the supplying of a DC voltage over a co-axial cable. Corrosion becomes a serious problem if the co-axial cable is damaged however so slightly.
Corrosion can be substantially eliminated by converting the DC voltage to an AC voltage, however the requirement of the provision of a DC voltage places strict limits upon this option.
DC voltage equipment can, in many instances be powered by an extremely low frequency AC voltage. The provision of a low frequency AC voltage by inverter circuits is not economically and technically efficient in many applications, however, because of the size and expense of the magnetics required at low frequency.
A voltage polarity switch, of the prior art, is configured as a bridge circuit in the schematic of FIG. 3. In this particular arrangement all of the illustrated FET switches Q.sub.1, Q.sub.2, Q.sub.3 and Q.sub.4 are driven by a PWM signal which is divided into PHASE-A and PHASE-B PWM series of pulses. This particular circuit is most profitably used with the generation of low frequency voltages with a substantially sinusoidal waveform. A sinusoidal waveform is not very suitable for powering DC voltage equipment with a low frequency AC voltage. Such a circuit is disclosed in "Power Electronics, Converters, Applications, and Design" by Ned Mohan et al, Wiley, c1989 pg. 115, 116.